Femoral stem for hip prosthesis

ABSTRACT

A femoral stem for hip prosthesis is disclosed comprising: a main body with a mainly longitudinal development and a generally wedge shaped, which is inserted into the femoral canal present in the body of the femur; a central body of a generally trapezoidal shape integral with the main body, located in the proximal zone of the femur; an appendix projecting from the central body provided with a terminal pin receiving spherical head of joint into the cotyle belonging to the prosthesis and inserted into the acetubular zone of the pelvic bone. The main body and the central body are defined by a shaped surface on one side and a mixtilinear surface on the opposite side from which a shaped notch is starting involving the central body and extending up to the proximity of the projecting appendix.

BACKGROUND OF THE INVENTION

The present invention relates to a femoral stem for hip prosthesis usedfor replacing the diseased natural joint of the hip with an artificialdevice taking its functions.

It is well known that in the orthopaedic field surgery of the hip jointis generally carried out for old people to treat pathologies such asarthrosis, arthritis or hip luxation generating a progressive wear ofthe hip joint.

It is well known that the hip joint connects the femur to the pelvis andcomprises the femur head engaged in the acetabulum which is a jointcavity on the outer face of the pelvic bone.

In the femur in addition to said femoral head there are the neckconnecting the head to the femoral body, having a rather longitudinaldevelopment, and a couple of protrusions, the greater and the lessertrochanter, arranged on opposite sides on the femoral body.

On the basis of the natural shape of the hip, the prosthesis to beimplanted comprises therefore a femoral part called stem and made ofmetal, generally of a titanium alloy or stainless steel, inserted into alongitudinal cavity made in the femoral body and an acetabular partcalled cotyle which is applied into a seat made in the pelvis.

The cotyle is generally consisting of two parts, the first made ofmetal, titanium or its alloys, and the other facing the stem made ofpolyethylene of high molecular weight or ceramics.

The cotyle is coupled with a spherical head also made of metal orceramics, which is fixed to the stem and is the pivot of the artificialjoint.

More particularly the femoral stem comprises a main body known also asshaft which is inserted in the femur body. Such a shaft ends at thetrochanter zone with a shoulder while continuous medially with a neckprojecting from the shoulder to the cotyle of the prosthesis and havinga terminal cone to which the above mentioned spherical head is applied.

The surgical operation to the patient's hip therefore consists inpreparing in the acetabular zone of the pelvis using proper sphericalmills, a seat in which the cotyle is then applied.

Then the neck is being cut with a saw, said operation being calledresection or ostheotomy and in the subsequent preparation through properrasps, of the internal canal of the femoral bone where the prostheticstem is then inserted.

Finally the spherical head is assembled with the femoral stem thusassembling the prosthesis and restoring the original configuration ofthe limb.

In this way once implanted, the femoral stem and the cotyle reproducealmost faithfully the original shape of the femur and the acetabulumrespectively, replacing the diseased parts, allowing a pain relief and arecovery of the joint function so as to allow patient to have a normallife for many years.

The prosthesis is anchored to the bone through two main methods, thefirst consisting in solidification during surgery of a liquid polymerthat in this way acts as a glue, connecting firmly the bone to theprosthesis. In this case a cemented prosthesis is obtained.

The prosthesis of most modern conception are however pressure appliedinto the bone, to which they are stably anchored through a naturalprocess of bone integration occurring with time, enhanced also by thefact that the metal stem is rather sanded, that is has a rough outersurface, thus increasing the anchoring ability of the bone to the stem.

The present invention intends to address this second kind of prosthesis.

A wide variety of embodiments of femoral stems for hip prosthesis arepresently available on the market, performing in different ways theobject aimed to.

The main limitation of the femoral stems of the prior art consists inthat they are rather rigid, above all in relation to the intrinsicelasticity characteristics of the bones of the human body and moreparticularly of the femur.

The application of a rigid metal body inside the femur generates indeedphysical decompensations to the person, above all in their areas ofgreater load transmission such as the cortical medial areas which becometherefore particularly critical areas.

The transmitted stresses are not properly absorbed by the cortical bonewhich is of the compact kind, with the consequence of causing pain tothe person but more particularly causing degeneration or evenprogressive disappearance of the bone in the less stimulated zones,among which a particular place is to be given to the Calcar locatedimmediately above the lesser trochanter.

Another limitation of the femoral stems of the prior art is their highdimension, more particularly at the shoulder, contributing to making theentire structure stiff.

The solution to make less cumbersome stems reducing the shoulderdimension, is not always possible because at this point the torsionalstability of the prosthesis is strongly reduced.

Moreover post operative radiographs show for these constructional typesa line of detachment between bone and prosthesis at the lateral shoulderof the stem.

It is clear that this aspect has a negative influence on the performanceof the prosthetic implant because the stem results to be partiallydetached from the femur, not to speak of the clear troubles that thissituation causes for the patient's health.

A further attempt to solve the problem of the stiffness of theprosthetic stem inside the femur is disclosed in the document EP 308297,wherein the stem has a notch of different shape made starting from thesurface connecting the shoulder with the neck and developed downwards inthe proximal zone.

Laboratory test of mechanical resistance show however that after havingbeing subject to a set of load cycles, this femoral stem fails byfatigue at the notch end, where a critical section is generated causedby an excessive concentration of stress. In this connection it is to berecalled that there is a European regulation (UNI7206) according towhich the femoral stem of hip prosthesis must withstand five millionsload cycles to be approved.

A second drawback of the stem disclosed in said document is that duringthe load only the part facing the lesser trochanter bends downwards,while the shoulder remains stationary thus preventing to keep thegreater trochanter reactive.

Another drawback of this stem is due by the fact that the notch asshaped allows that chips produced by rubbing of the spherical headagainst the cotyle, to deposit by gravity inside through the upperopening, this leading with time to prosthesis failure.

SUMMARY OF THE INVENTION

The present invention aims at overcoming the above mentioned drawbacksand limitations of the prior art.

More particularly it is a first object of the invention to provide afemoral stem for hip prosthesis that under low conditions allows tostress the femur in a more uniform way relative to equivalent stems ofthe prior art.

It is another object to provide the femoral stem that does not allowgeneration of sides for accumulation of particles and chips producedinside the joint.

Said objects are attained by a femoral stem for hip prosthesis thataccording to the contents of the main claim comprises:

-   -   a main body with mainly longitudinal development and with a        generally wedge shape, adapted to be inserted into the femoral        canal of the femur body;    -   a central body of a generally trapezoidal shape integral with        said main body, adapted to be received in the proximal zone of        said femur;    -   an appendix projecting from said central body, provided with a        terminal pin adapted to receive spherical join head in the        cotyle belonging to said prosthesis and inserted in the        acetabular zone of the pelvic bone,

and wherein said main body and said central body are defined by a shapedsurface on one side and a mixtilinear surface on the opposite side fromwhich a shaped notch is starting, involving said central body andextending up to the proximity of said projecting appendix.

According to a preferred embodiment of the invention, the mixtilinearsurface comprises a first generally straight surface belonging to thecentral body, and a second generally straight surface belonging to themain body, connected to the first surface through a generally convexradiused zone from which the shaped notch is starting.

Advantageously the femoral stem of the invention stimulates the femoralbone in a more uniform way relative to equivalent stems of the priorart.

More particularly, the femoral stem stimulates efficiently the femurarea called the calcar, that otherwise is progressively but invariablydegenerated, thus warranting the reactivity and consequently its lifefor a more extended period of time in comparison with the stems of theprior art.

This is not to the detriment of the greater trochanter of the femur,which is also stimulated because the entire central body bends downwardswhen the stem is loaded.

Still advantageously the femoral stem of the invention keeps a ratherwidened geometry enhancing to obtain an optimal primary stability, to beunderstood as the torsional resistance during the period immediatelyafter the surgery.

Again advantageously the invention allows to reach an optimal secondarystability to be understood as the ability of the bone to integrate thefemoral stem in the subsequent phases after the post operative courseduring the patient's normal life.

In advantageously way moreover the shaped notch realized to make thestem more subject to deformation and therefore compatible with theintrinsic characteristics of the femur, is developed upwards andtherefore prevents the chips falling by gravity to be deposited insideit.

Moreover the shaped notch made at a discontinuity zone of one of thelateral surfaces defining the stem, namely where the inclination of thelateral profile of the stem is changing, divides the portion of thefemoral stem anchored to the cortical bone from the portion anchored tothe spongy bone.

This prevents that in proximity of the notch a stress is generated dueto the point contact of the stem with the cortical bone.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and advantages, and others that will be pointedout in the course of the present description, will be better understoodby the description of preferred embodiments of the invention given asillustrative but not limiting examples and shown in the accompanyingsheets of drawings in which:

FIG. 1 is a partially sectioned view of the femoral bone and the hipjoint;

FIG. 2 is a partially sectioned of the femoral stem of the inventioninserted in the femoral bone of FIG. 1;

FIG. 3 is a view of the femoral stem of the invention;

FIG. 4 is an enlarged detail of the stem of FIG. 3;

FIG. 5 is an executive variation of the femoral stem of FIG. 3;

FIG. 6 is an enlarged detail of the stem of FIG. 5.

FIG. 1 shows the femoral bone generally indicated with F and its jointin the acetabular zone generally indicated with E, and belonging to thepelvic bone O to form the hip A.

DESCRIPTION OF THE INVENTION

One can also see the femoral body C, with straight development, the headT of femur F articulated in the acetabular zone E and connected to thefemoral body C through the neck L, the lesser and greater trochanterindicated with P and G respectively, the proximal zone Z comprisedbetween the two trochanters and the femoral canal N in the femoral bodyC in which the prosthetic stem is inserted.

In this connection it is adviseable to point out the internal structureof the femur F in which there is the spongy bone J and mainly located inthe diaphises portion of the femur F.

One can also see in heavy lines, the osteotomy line M along which thehead T of the femur F is being cut to allow to work the femoral canal N.

In FIG. 2 the femoral stem of the invention generally indicated with 1and belonging to prosthesis R, is shown in the applied condition, thatis inserted inside the femur F.

One can see that the femoral stem 1, preferably but not necessarily madeof titanium alloy, comprises a main body 2, with a mainly longitudinaldevelopment and with a generally wedge shape, which is inserted into thefemoral canal N of the body C of femur F.

Therefore the femoral stem 1 comprises a central body 3 of a generallytrapezoidal shape integral with the main body 2 which is located in theproximal zone Z of femur F.

Finally the femoral stem 1 comprises an appendix 4 projecting from thecentral body 3, provided with a terminal pin not visible in FIG. 2 butseen in FIG. 3 where it is indicated with 5, receiving the sphericalhead S of joint in the cotyle I belonging to the prosthesis R insertedin the acetubular zone E of the pelvic bone and provided with a portionB for instance made of polyethylene, metal or ceramics, facing thefemoral stem 1.

According to the invention, the main body 2 and the central body 3 aredefined by a shaped surface 6 on one side and a mixtilinear surface 7 onthe other side, from which a shaped notch 8 is starting, involving thecentral body 3 and extending up to the proximity of the projectingappendix 4.

In the present embodiment the shaped surface 6 as a concave-convexprofile in longitudinal section.

As shown in FIG. 3, the mixtilinear surface 7 consists of a generallystraight first surface 7 a belonging to the central body 3 and a secondgenerally straight surface 7 b belonging to the main body 2 connected tothe first surface 7 a through a generally convex radiused zone 7 c fromwhich the shaped notch 8 is starting.

Therefore at the radiused zone 7 c a change of inclination of theprofile of the femoral stem 1 occurs, so that the extension of thesecond surface 7 b defines with the first surface 7 a an acute angle α.

Said change of inclination of the lateral profile of the femoral stem 1at the connection zone between the main body 2 and the central body 3 isdictated by the dual requirement to obtain stem of a widened shape atthe proximal zone Z to warrant a good torsional stability from one side,and to allow a saving in bone removal of femur F in the zone adjacent tothe greater trochanter G from the other side to insert the femoral stem1 inside the femur F.

In view of the various muscles present in said zone connected to thefemur F such as for example those very important ones of the glutei, itis clear then the more one succeed to preserve this zone and the betterwill be the post operative recovery of the patient. This is obtainedjust by making a mixtilinear surface 7 as taught by the presentinvention.

Moreover the change of inclination of the lateral profile of the femoralstem 1 occurs at a zone that when the insertion of the femur F iscompleted, is generally located in contact with the spongy bone J of thefemur which is more internal and almost exclusively present in theproximal zone Z. This allows to avoid that at the shaped notch 8 thereis stress concentration that otherwise would occur if the notch would bein contact with the cortical bone K.

In FIG. 3 one can also see that the shaped notch 8 divides the centralbody 3 into a first zone 3 a located generally facing the greatertrochanter G of the femur F and a second zone 3 b located generallyfacing the lesser trochanter P of the femur F.

The first zone 3 a and the second zone 3 b are mutually connected by abridge 3 c comprised between the shaped notch 8 and the radiused surface9 between the projecting surface 4 and the mixtilinear surface 7.

The shaped notch 8 whose shape comes from tests with finite elementsmethod, consists of a concave-convex continuous surface 8 a defining aprofile 8′ generally having a shape of a half slot as shown in FIG. 4.

More particularly the profile 8′ is preferably but not necessarilyconstituted by a first stretch 8 b connected to the mixtilinear surface7 with a generally constant cross section and a second stretch 8 cextending until under the bridge 7 c with a widened crossed section.

FIG. 5 shows an executive variation of the femoral stem of the inventionnow generally indicated with 100, which is different from the precedingembodiment because the first zone 101 a of the central body 101 isprovided with an outer couple of longitudinal fins indicated with 103and 104 respectively, extending generally for the entire length L₁ ofthe first zone 101 a.

As shown in FIG. 6 according to the preferred embodiment now describedthe longitudinal fins 103, 104 have a cross sectional conical profile103′, 104′ and are arranged along corresponding longitudinal axis Y, Y′parallel to each other and generally parallel to the first surface 102 aof the mixtilinear surface 102 defining the central body 101 from oneside.

It is clear that according to another executive variation, the firstzone of the central body may comprise any number of longitudinal fins asa function of the dimensions of said central body depending in turn bythe configuration of the femur in which the femoral stem is beinginserted.

Such longitudinal fins have the purpose to enhance the bone integrationstimulating the spongy bone with which they enter exclusively in contactin view of their arrangement on the stem without damaging it to muchthanks to their rounded profile.

Moreover their development line is rather advantageous when the stem 100is inserted in the femur F with a curved path defining the type ofinsertion known in the art as banana insertion, which is not coincidentwith the vertical direction parallel to the axis Z′ of the femur F.

This insertion method well known in the surgery methods, allows afurther bone saving in the zone adjacent to the greater trochanter G.

During this kind of operations, the longitudinal fins 103, 104 remainalways tangent to the curved path of insertion of the femoral stem 100into the femur F.

In this way the fins do not rub and remove spongy bone J so as not tocreate a marked hollow zone between the femoral stem 100 and the femoralbone F which would be hard to fill with time by the natural process ofbone integration as it occurs however with the femoral stems of theprior art.

This latter stems are indeed provided with outer longitudinal fins thatin surgical operations of banana insertion of the stem into the femur,cancel the object for which they were provided, namely to allow a betteranchorage of the femur to the stem, because said fins are arranged alonga vertical direction generally parallel to the main developmentdirection of the stem.

Operatively the surgery of application of the prosthesis are to the hipA of the patient, more particularly of the femoral stem 1 of theinvention, consists initially in reception of the head T of femur F andthe subsequent preparation by proper rasps of the femoral canal N.

To obtain the most possible advantages of the invention, the seat of thefemoral stem 1 is advantageously obtained in some zones without removingthe spongy bone J as it occurs in most surgical methods now used inorthopedic surgery but pressing it against the cortical bone K.

This allows to stimulate equally the cortical bone K making it reactiveand therefore adapted to grow and strengthen besides keeping the spongybone J enhancing the ability of bone integration to the femoral stem.

It is also known that even a partial saving of removal of spongy bonedecreases the blood loss and makes the post operative recovery of thepatient easier.

The surgery continues with removal of the zone comprised between thegreater trocanter G and the neck L already removed, to allow insertionof the femoral stem 1 into the femoral canal N of femur F indifferentlyalong a vertical direction or curved path.

Then one proceeds to the application of the spherical head S to theterminal pin 5 of the femoral stem 1 thus allowing the joint with cotyleI previously applied to the acetubular zone E and completing theassembly of prosthesis R of the hip A.

Under load conditions, the second zone 3 a of the central body 3 bendsdownwards thus stimulating the zone of femur F facing it, moreparticularly the calcar subject to degeneration with femoral stems ofthe prior art because it is not sufficiently stimulated.

Thanks to the shaped notch 8 making the femoral stem 1 less inert, underload conditions even the first zone 3 a of the central body 3 is pulleddownwards thus stimulating advantageously the greater trochanter G.

Therefore the femoral stem 1 as much as possible being of metalmaterial, results to be less rigid in comparison with equivalent stemsof the prior art, still not compromising its primary and secondarystability indispensable for the correct anchorage and integration withfemur F.

As a matter of fact, the peculiar characteristics of the femoral stem ofthe invention are those to be provided with a certain degree ofelasticity making it more integral with the femur to stimulate thecrucial parts and at the same time not to hinder or compromise theprocess of anchorage and bone integration to the stem because of anincorrect and excessive mobility inside the femur.

It is clear that these aspects confer to the femoral stem of theinvention an absolute level of peculiarity and efficiency that cannot bereached with the stems of the prior art.

On the basis of the foregoing it is therefore understood that thefemoral stem of the invention attains all the previously mentionedobjects and advantages.

In the executive stage modifications made be made to the femoral stem ofthe invention for example consisting in a different profile of theshaped notch.

In addition even the surfaces defining the femoral stem may have adifferent shape from the above described one, without impairing theadvantage given by the present invention.

All the variations described and cited but not illustrated in theaccompanying sheets of drawings, when falling in the scope of theappended claims, should be considered protected by the present patent.

1. A femoral stem for hip prosthesis, comprising: a main body withmainly longitudinal development and with a generally wedge shape,adapted to be inserted into the femoral canal present in a body of afemur; a central body of a generally trapezoidal shape integral withsaid main body, adapted to be located in a proximal zone of said femur;and an appendix projecting from said central body, provided with aterminal pin adapted to receive a spherical head of a joint in a cotylebelonging to said prosthesis and inserted in an acetabular zone of apelvic bone, wherein said main body and said central body are defined bya shaped surface on a medial side of the femoral stem and by a surfacehaving a mixtilinear profile on an opposite lateral side, wherein saidcentral body includes a shaped notch in the form of an open-ended slot,wherein the shaped notch is a solitary notch, wherein said notch,wherein said notch includes an open end at the mixtilinear profile ofsaid main body and extends generally toward the projecting appendix,said shaped notch passing through a thickness of said central body froman anterior side to a posterior side of the femoral stem, wherein thecentral body includes a first zone, arranged generally to face a greatertrochanter of said femur, and a second zone, arranged generally to facea lesser trochanter of said femur, and wherein said first zone and saidsecond zone are joined at a bridge portion disposed adjacent a closedend of the notch at the posterior side of the femoral stem.
 2. Thefemoral stem according to claim 1, wherein said shaped notch has asmooth concave-convex continuous inner surface defining.
 3. The femoralstem according to claim 1, wherein said mixtilinear profile consists ofa first generally straight profile at said central body, and a secondgenerally straight profile at said main body approaching said firstprofile at the opening of said shaped notch.
 4. The femoral stemaccording to claim 1, wherein the first and second zones are connectedonly at the bridge, and are otherwise completely separated by the shapednotch.
 5. The femoral stem according to claim 3, wherein an extension ofsaid second profile in the direction of the first profile defines withsaid first profile an acute angle.
 6. The femoral stem according toclaim 2, wherein said inner surface of said shaped notch has a profilethat consists of a first portion beginning at the opening of the notchhaving a generally constant cross section, and a second portioncontinuing from the first portion and extending until below said bridge,having a widened cross section.
 7. The femoral stem according to claim1, wherein said first zone of said central body includes at least oneexternal longitudinal fin generally extending for the entire length ofsaid first zone.
 8. The femoral stem according to claim 7, wherein saidat least one fin is arranged along a longitudinal axis generallyparallel to said first profile of said mixtilinear profile.
 9. Thefemoral stem according to claim 7, wherein said at least one fin has across sectional conical profile.
 10. The femoral stem according to claim1, wherein said shaped surface has a concave-convex profile inlongitudinal section.